Shape-stabilization of organic phase change materials as mechanically stable silica boards with high latent heats synthesized via sol-gel route

Monolithic shape-stabilized phase change materials (ss-PCM) are of great interest for energy -saving construction materials, solar-water heating systems and passive cooling systems of bat-teries and photovoltaic cells. However, the application range of ss-PCMs is currently limited by either low mechanical stabilities or low heat storage capacities. Therefore, we recently developed ss-PCMs based on silica and butyl stearate (BS) with high compressive strengths (1.2 MPa, 10 degrees C), but only moderate latent heats (-100 J/g). Here, we focus on validating the applicability of our porogen-assisted sol-gel process to other organic PCMs apart from butyl stearate. To increase the latent heat of ss-PCMs, we create a mixture of 70% hexadecane (HD) or octadecane (OD) with 30% BS and combine this PCM mixture with our novel-sol-gel route to synthesize ss-PCMs with high compressive strengths (1.2 MPa, 5 degrees C) as well as high latent heats (160 J/g). Due to the well -interconnected silica matrix confining the paraffins, the ss-PCMs have 70% higher thermal con-ductivities (0.43 W/mK, 5 degrees C) than pure paraffins, high shape-stabilities (-100%), high PCM loadings (83 wt%), and high chemical and long-term stabilities (>3000 thermal cycles). More-over, they are thermally stable up to-170 degrees C with PCM melting points of 17 degrees C and 22 degrees C. We also synthesize ss-PCMs with pure polyethylene glycol 600 (PEG600) and pure acetamide (Ac) to analyze the effect of non-paraffinic PCMs on the physicochemical properties of ss-PCMs. These ss-PCMs have a-30% lower latent heat than expected because the PCM either reacts strongly by hydrogen bonding with the silica (PEG600) or was chemically degraded under the basic condi-tions during gelation (Ac). In contrast to the low compressive strength of shape-stabilized PEG600 (35 kPa, 5 degrees C), the ss-PCMs based on Ac have high compressive strengths (9.6 MPa, 5 degrees C). Moreover, they are translucent above the PCM melting point due to a nanoscopic silica matrix. The high supercooling effect (37 degrees C) and the moderate long-term stability (3000 thermal cycles, 20% lower latent heat) of shape-stabilized Ac restrict its application to special, glassy materials.